A conventional hydraulic machine such as, for example, a hydraulic excavator comprises a hydraulic pump forming a source of hydraulic fluid and driven by rotational power of an engine, a plurality of actuators such as a running motor, a boom cylinder and so on driven by the hydraulic fluid supplied from the hydraulic pump, a plurality of control valves corresponding respectively to the actuators, such as a running-motor control valve, a boom-cylinder control valve and so on for controlling respectively flow directions and flow rates of the hydraulic fluid supplied to the actuators, and a control device for giving control signals respectively to drive sections of the respective control valves for the actuators in response to operational directions and amounts of operation of operating levers arranged correspondingly respectively to the actuators, thereby controlling operations of the respective control valves to bring the actuators respectively to requisite operational states (operational directions and operational speeds) in accordance with various workings.
Generally, the conventional hydraulic driving apparatus is so designed that the maximum opening degrees of the control valves determined at the initial specification cannot subsequently be altered. Accordingly, when the hydraulic driving apparatus is operated in such a state that the operational speeds of the respective actuators are maximum, hydraulic fluid of a predetermined discharge quantity determined by the rotational speed of the engine is supplied from the hydraulic pump to the actuators through the control valves, during a period for which the rotational speed of the engine driving the hydraulic pump is constant, so that the maximum operational speeds of the respective actuators are maintained constant. When it is desired to alter the maximum operational speeds of the actuators, the rotational speed of the engine must be altered to vary the discharge quantity of the hydraulic pump, that is, flow rates of the hydraulic fluid supplied respectively to the actuators. With such a way of altering the maximum operational speeds of the actuators, however, it is impossible to alter the operational speeds of only specific one or some of the plurality of actuators, because the hydraulic fluid from the hydraulic pump is supplied to all the actuators under the same condition.
In the hydraulic machine such as a hydraulic excavator or the like, however, there may occur a case where it is desired to slow down the operational speed of only a specific one of the actuators in accordance with the working contents in view of the actual working. For example, for the hydraulic excavator, there may be a case where such an operation as to draw the bucket horizontally is carried out for ground leveling. In such a case, relatively high working accuracy is required to draw the bucket horizontally toward this side, so that it is desired to slow down the operational speed of only the bucket cylinder or the operational speed of only the arm cylinder. In the conventional hydraulic driving apparatus, however, it is impossible to slow down the operational speeds of only specific some of the plurality of actuators, as described previously. That is, if the rotational speed of the engine is lowered, the operational speeds of all the actuators such as the boom cylinder, the arm cylinder and the like are slowed down uniformly. In this manner, if the rotational speed of the engine is lowered in an attempt to improve the working accuracy, the operational speeds of all the actuators such as the bucket cylinder, the arm cylinder and so on are slowed down simultaneously, resulting in lowering of the working efficiency. Conversely, if the rotational speed of the engine is kept high in an attempt to improve the working efficiency to set the operational speeds of all the actuators to their respective rapid states, this leads to lowering of the working accuracy. Thus, it has been impossible for the conventional hydraulic driving apparatus to satisfy the working accuracy and the working efficiency simultaneously. Further, also in crane working in which a load is hung by a bucket and is moved up and down, there arises such a problem similar to that described previously that operational speed of a specific actuator is required to be slowed down for the purpose of prevention of swing of load.
As the prior art of a hydraulic driving apparatus for solving a problem similar to that discussed above, there are JP-A 60-65835, JP-U 60-111958 and JP-U 61-89060. JP-U 60-65835 discloses a method of operating a construction machine, in which a switch enabling minute speed regulation is provided at an operating lever for regulating an amount of displacement of a spool of a main valve, and an output characteristic of an operational amplifying circuit is altered on the basis of an output signal from the switch, to improve operability at the minute speed due to the operating lever. JP-U 60-111958 discloses an operating circuit of an actuator for a construction machine. In the operating circuit, a valve, in which a gain relating to a drive signal is varied depending upon rotational speed of an engine, is used in substitution for a conventional slow return valve provided in a return line from a hydraulic cylinder, to prevent a pulsating phenomenon and cavitation in the hydraulic cylinder, and to achieve energy saving. JP-U 61-89060 discloses a working-machine operating apparatus for a hydraulic power excavator, in which, from a plurality of operational modes set beforehand so as to be capable of optionally selecting working-machine speed, one operational mode can be selected in accordance with the working contents. However, none of the prior art references discloses a technique capable of solving the previously discussed problem.
It is a first object of the invention to provide a hydraulic driving method of a hydraulic machine, in which, in order to solve the above problem, respective operations of a plurality of hydraulic actuators are controlled in accordance with working contents, thereby being capable of improving both working accuracy and working efficiency.
It is a second object of the invention to provide a hydraulic driving apparatus for a hydraulic machine, in which, in a hydraulic machine equipped with a plurality of hydraulic actuators, operational-speed characteristics of at least one of the plurality of hydraulic actuators can be altered in accordance with working contents without exerting an influence upon the remaining actuators, thereby improving both working accuracy and working efficiency.